MOVEMENTS, ETC., OF FEESH-WATEll PLANARIANS. 683 



experiment was repeated many times on this piece and on 

 others cut in the same way. The same method of righting 

 was always observed. After the first few times the turn 

 is made in this way ; it is done more quickly at each succes- 

 sive trial. 



This experiment leads to the same conclusion regarding 

 the mechanism of the righting reaction as did the previous 

 one. It affords another aud more striking example of regu- 

 lation in reactions. The piece attains the end (normal 

 position) by a reaction which it undoubtedly never had 

 occasion to practise before. 



Isolated longitudinal halves of the body react in the same 

 way as did the piece described in the preceding experiment. 

 They right themselves by folding under the edge, and then, 

 by violent contraction, drawing the rest of the body up over 

 it. There is no trace of the spiral righting reaction. 



A specimen cut in the manner shown in Fig. 41 shows a 



Tig. 41. — Operation diagram (see text). 



very peculiar righting reaction. When placed dorsal side 

 down the portion posterior to the median longitudinal 

 slit immediately gives the spiral righting reaction, and drags 

 the two passive anterior pieces over. The process is slow but 

 very characteristic, so that there is no doubt of the nature of 

 the reaction. This shows that in that part of a single piece 

 of a worm where the necessary mechanism is present we get 

 the spiral righting reaction, while in other parts it docs not 

 appear. 



The same point can be brought out by splitting a worm 

 longitudinally from the posterior end up to a point near the 

 head. The complete anterior part of such specimens gives 

 the normal spiral reaction, while the posterior ])arts remain 

 passive so far as this reaction is concerned. 



A considerable number of dilferent experiments were per- 



